Capacitive gas sensors are known in the art, a particular example being in the measurement of water vapor (relative humidity). There are a number of configurations associated with these sensors. One configuration uses interdigitated coplanar electrodes of opposite polarity covered by a gas sensitive material in which increasing gas concentration causes an increase in the dielectric constant of such material thereby increasing the dielectric coupling between planar electrodes and thereby increasing the effective capacitance between the electrodes. In the case of the interdigitated electrodes, both electrodes are underneath the top surface of the gas sensitive material, and the dielectric coupling between the planar electrodes occurs by field fringing effects.
Another configuration employs parallel plate-like electrodes with a layer of gas sensitive material between them such that changing gas concentration changes the dielectric constant of the gas sensitive material and changes the capacitance of the parallel plate capacitor. A parallel plate configuration described in FR2750494 (U.S. Pat. No. 6,450,026) has a top electrode comprised of a highly porous conducting polymer that allows the diffusion of the selected gas through the electrode and into the gas sensitive material. This top electrode material is processed so that it is tightly bonded to the gas sensitive material and is chemically inert and environmentally robust. FR2750494 and U.S. Pat. No. 6,450,026 are incorporated by reference herein in their entirety.
The capacitance of a capacitive gas sensor is a function of gas concentration, and the capacitance is measured by associated electronics capable of exciting the sensor electrically. The cost of manufacture of the capacitive gas sensors is associated with the physical size of the sensor and the associated electronics, hence it is desirable to provide capacitive gas sensors as small as possible while still achieving desired accuracy and signal to noise ratio. As the size of gas sensitive capacitors is reduced, the gas sensitive capacitors become increasingly susceptible to signal degradation associated with stray capacitances, including parasitic capacitances found in interconnections and in the associated electronics. One way to reduce the effects of parasitic capacitances when using smaller capacitors is to locate the associated electronics as physically close to the sensor as possible.
Along with reducing the size and therefore the cost of manufacture of the capacitive gas sensors, it is desirable to decrease the size and cost of the associated electronics. Reduced cost of manufacture of the associated electronics can be achieved through the use of application specific integrated circuits (ASICS) which provide all necessary functionality in a small low cost configuration.
There are commercial devices available in which interdigitated coplanar capacitor electrodes are disposed on top of a section of an ASIC and a gas sensitive material layer is disposed on top of the coplanar sensor electrodes to form a gas sensor. This configuration for ASICs with interdigitated capacitor electrodes has a disadvantage in that if the interdigitated capacitor electrodes are directly over active circuitry in the ASIC, coupling and interference is likely. This results in a need for a larger silicon area to accommodate the sensor electrodes so they are not over active circuitry. Notably, the interdigitated electrodes cannot be shielded from the circuitry-induced stray coupling signals by the addition of a conductive layer intermediate the electrodes and underneath because such a conductive layer would significantly increase the baseline capacitive coupling between the interdigitated sensing electrodes. Since the signal generated by changing gas concentrations in the gas sensitive layer is measured as a changing percentage of capacitance, increasing the baseline capacitance will lower the sensitivity of the device. Another drawback to ASICs with interdigitated electrodes is the undesirable sensitivity of the inter electrode capacitance to foreign material on the top of the gas sensitive material layer. For example, water droplets or small metal particles on the surface of the gas sensitive material layer can significantly alter the dielectric coupling between the electrodes by distorting fringing electric fields generated by the electrodes.
An alternative device has two separate chips: one chip has a gas sensing capacitor built atop an appropriate substrate and the second chip suitable circuitry. This two chip solution has the advantage of decoupling the production yields, the processes, and the substrate materials used to produce each part. However, these chips must be electrically interconnected using flip chip or wire bonding technology, both of which affect the behavior of the sensing capacitor. Further the cost of electrical and mechanical packaging is greater than the vertically integrated configuration.
Therefore, there is a need for a smaller and more effective capacitive gas sensor constructed directly on top of an appropriate semiconductor circuitry.